Preparation and photocatalytic properties of quartz/gold nanostructures/TiO₂ lamellar structures

Abstract

A novel class of visible light activated titanium oxide (TiO₂) photocatalytic surfaces were prepared by depositing gold nanostructures (Au NSs) onto quartz substrates and subsequent coating with 20–200 nm thick TiO₂ thin films. The Au NSs were obtained by thermal evaporation and post-annealing of 4 nm thick gold films, while the TiO₂ thin layers were deposited using reactive magnetron sputtering. The resulting interfaces were characterized using scanning electron microscopy (SEM) and UV-vis absorption spectroscopy. UV-vis spectra showed strong absorption peaks between 570 and 680 nm due to the plasmonic response of the lamellar structure. The photocatalytic activity of the interfaces for the oxidative degradation of rhodamine B was evaluated under ultraviolet and visible light illumination. The interface coated with 100 nm TiO₂ exhibited pronounced photocatalytic activity under UV light irradiation compared to substrates with thinner or thicker TiO₂ films or without TiO₂ coating. A quartz/Au NSs/100 nm TiO₂ interface showed higher photocatalytic activity compared to quartz/100 nm TiO₂, while retarded photodegradation of rhodamine B is observed with quartz/Au NSs coated with thinner or thicker TiO₂ films. The results clearly highlight the positive effect of the Au NSs on the performance of thin TiO₂ photocatalysts. Furthermore, the deposition of TiO₂ on top of Au NSs improves the poor stability encountered in TiO₂/Au NSs photocatalytic systems.

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1 Introduction

Semiconductor-based photocatalysts such as titanium dioxide (TiO₂) and zinc oxide (ZnO) have been widely employed for the removal of highly toxic and nonbiodegradable pollutants present in air and water over the last decade.^1–3 The principle of photocatalysis is based on the generation of photoelectrons (e⁻) in the conduction band and holes (h⁺) in the valence band of the semiconductor upon exposure to light of sufficient wavelength. The e⁻ and h⁺ charges migrate to the surface of the semiconductor where they serve as redox sites for the destruction of surface-adsorbed pollutants.⁴ The photocatalytic activity is mainly controlled by four factors: (i) the wavelength of light, (ii) its intensity, (iii) the rate of diffusion of the photogenerated charges to the surface of the semiconductor, and (iv) the rate of surface oxidation. In the case of TiO₂, the crystal structure as well as porosity of the film and the amount of surface hydroxyl groups have to additionally be considered.^5,6 To prepare materials with optimal photocatalytic properties, parameters such as surface area, concentration of defects and crystallinity of the material have to be controlled.^7,8 Most TiO₂ thin film photocatalysts are prepared by using wet-chemical processes such as the sol–gel method, mainly due to the advantages of simple equipment, low cost and the ease with which large or complicated surfaces can be coated.^9–11 Sputtering methods have shown several advantages over wet-chemical processes namely the ease and accuracy to control the film thickness on the nanometer scale as well as the technical simplicity.^10,12,13 The group of Hupp et al. has shown recently that atomic layer deposition can be used to deposit pinhole-free crystalline TiO₂ layers of 7.7 nm directly onto fluorine doped tin oxide interfaces.¹⁴ This approach was used to protect surface linked silver nanoparticles from oxidation processes.¹⁴

The interest of doping TiO₂ with noble metals is to enhance its photocatalytic performance.^2,8,15–19 Recent studies evidenced that the photocatalytic properties of such hybrid materials depend highly on the preparation method, in particular on the conditions of gold deposition, gold loading, particle size and shape and on the storage conditions.^18,20,21 In all these studies, the metallic particles were deposited on TiO₂. However, the poor adhesion of noble metal nanoparticles to most inorganic oxide substrates can result in morphological changes and aggregation upon exposure to solutions limiting the use of such photocatalytic platforms.^22,23 Ourselves^22,24–31 and others³² have shown that metallic nanostructures can be stabilized by post-coating them with thin dielectric overlayers. Such hybrid structures show localized surface plasmon resonance (LSPR) bands, where the position of the plasmonic wavelength (λ_max) depends on the thickness and the refractive index of the overlayer.^24,26,33 In addition, the interaction between the Fabry–Perot modes established inside the dielectic overlayer with the plasmonic of the nanostructures leads to a thickness-dependent shift of λ_max.³⁴

As the surface plasmon of metallic particles allows electron exchange to and from TiO₂ nanocrystallites, the influence of TiO₂ coated Au NSs, deposited on quartz substrates, on the photocatalytic performance for rhodamine B oxidation is investigated in this paper. A quartz/Au NSs/100 nm TiO₂ interface showed higher photocatalytic activity compared to quartz/100 nm TiO₂, while retarded photodegradation of rhodamine B is observed with quartz/Au NSs coated with thinner or thicker TiO₂ films. The results clearly highlight the positive effect of the Au NSs on the performance of thin TiO₂ photocatalysts. Furthermore, the deposition of TiO₂ on top of Au NSs improves the poor stability encountered in TiO₂/Au NSs photocatalytic systems.

2 Experimental

2.1 Materials

Acetone, 2-propanol, sulfuric acid and rhodamine B were purchased from Sigma-Aldrich.

2.2 Formation of gold nanostructures on quartz

Quartz slides (20 × 50 × 1 mm, ACM, France) were first cleaned in 2-propanol and acetone in an ultrasonic bath at room temperature, rinsed copiously with Milli-Q water and dried under a stream of nitrogen.

The clean substrates were then transferred into an evaporation chamber. Gold nanostructures deposition was carried out by thermal evaporation of 4 nm thick gold films using a MEB 550 S evaporation machine (Plassys, France). Post-deposition annealing of the metal-covered quartz slides was carried out at 800 °C for 10 min under nitrogen atmosphere using a rapid thermal annealer (Jipelec Jet First 100), leading to circular-shaped dense nanostructures.

2.3 Deposition of titanium oxide (TiO₂) thin films

Titanium dioxide layers were deposited onto quartz/gold nanostructures (quartz/Au NSs) using DC magnetron reactive sputtering. The deposition chamber (Alliance Concept AC450) contains a pure Ti target (3 inches in diameter, purity 99.995%) and the following parameters were used: distance between the target and the quartz/Au NSs interface = 131 mm; pressure = 0.5 Pa, DC power = 120 W, current = 300 mA; potential on target: 412 V, gas flow: argon/oxygen (50/1.5 sccm), deposition rate: 1 nm min⁻¹ and without intentional heating.³⁵

2.4 Annealing

The whole samples underwent a single-step annealing process in a muffle-furnace. They were heated to 600 °C for 72 h in air. A heating ramp of 6 °C min⁻¹ was used. Cooling was run very slowly to avoid cracks (2 °C min⁻¹).

2.5 Photocatalytic activity measurements

The photocatalytic activity of the TiO₂ interfaces was investigated using 2 mL aqueous solution of rhodamine B (RhB) with an initial concentration of 5 × 10⁻⁶ M. In a typical experiment, the photocatalyst was put into a quartz cuvette filled with the RhB solution. An initial absorption spectrum between 300–800 nm was recorded. Thereafter, the assembly was irradiated at λ = 365 nm or λ > 420 nm.

2.5.1 Photodegradation of RhB under UV light irradiation. The RhB aqueous solution was irradiated at room temperature in air using a UV fiber lamp (Spot Light Source 300–450 nm, L9588-01, Hamamatsu, Japan) equipped with a bandpass filter centered at 370 ± 2 nm in a 1 cm spectrometric cuvette. The intensity of the light was measured using a C6080 Series UV Power Meter (Hamamatsu, Japan) and was determined as being 0.5 W cm⁻².

2.5.2 Photodegradation of RhB under visible light irradiation. The RhB aqueous solution was irradiated in air at room temperature using a visible fiber lamp (Spot Light Source 400–700 nm, L9566-03, Hamamatsu, Japan, with a filter having a net cut at λ = 420 nm, to suppress the light with wavelength shorter than 420 nm) in 1 cm spectrometric cuvette. The intensity of the light was measured using a PM600TM Laser Fiber Power Meter (Coherent Inc, USA) and was determined as being 0.5 W cm⁻².

2.6 Instrumentation

UV-vis spectrometer. Absorption spectra were recorded using a PerkinElmer Lambda UV-vis 950 spectrophotometer in polystyrene cuvettes with an optical path of 10 mm. The wavelength range was 250–800 nm.

Scanning electron microscopy (SEM). SEM images were obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field emission emitter, a high efficiency In-lens SE detector and an energy dispersive X-ray (EDX) analysis device.

X-Ray diffraction (XRD). X-Ray Diffraction patterns were recorded with a Seifert 3003 PTS diffractometer in a θ–2θ parallel beam configuration. From a copper anticathode (40 kV, 40 mA), a parabolic multilayer and a Ge(220) monochromator, both mounted on the primary beam, reflected an intense and parallel beam of Kα₁ radiation (with λ(Kα₁) = 0.154056 nm). 2θ angle was swept from 10 to 80° by 0.01° steps. Each acquisition step lasted 4 s for a better signal-to-noise ratio.

After annealing, TiO₂ indexation was obtained in reference to the joint committee in powder diffraction standards (JCPDS) file 71-1167 (anatase tetragonal TiO₂ structure with lattice parameters a₀ = b₀ = 0.37892 nm and c₀ = 0.9537 nm).

3 Results and discussion

Fig. 1 displays schematically the two different interfaces investigated in this work. In one case, a TiO₂ film with a thickness between 20–200 nm is directly deposited onto quartz slides, using reactive magnetron sputtering in the DC mode (Fig. 1A). The other interface consists of a quartz slide decorated first with gold nanostructures (Au NSs),^26,28 and post-coating the Au NSs with 20–200 nm TiO₂ films (Fig. 1B).

Fig. 1 Schematic representation of the different interfaces investigated in this work: (A) quartz coated with 20–200 nm thick TiO₂, (B) quartz decorated with gold nanostructures coated with 20–200 nm thick TiO₂ films.

3.1 Characterization of quartz/TiO₂ interfaces

Fig. 2 shows SEM images of quartz substrates coated with increasingly thick TiO₂ layers. Increasing the thickness of the TiO₂ overcoating layer leads to a slight increase of the TiO₂ crystallites.³⁶ The crystallinity of the films was investigated by XRD. A common feature in all XRD patterns is the wide diffusion hump around 2θ ≈ 21° originating from the quartz substrate (Fig. 3). Fig. 3a displays an XRD pattern of a 200 nm thick TiO₂ film deposited on a quartz slide. The as-deposited TiO₂ film shows no diffraction peaks due to its amorphous structure. After thermal annealing at 600 °C for 72 h, diffraction peaks corresponding to the (101), (004) and (105) anatase planes are observed (Fig. 3b). The intensity ratio of the (004) peak to the (101) peak is much higher (1.0) compared to a random distribution (0.18). Therefore, TiO₂ films crystallize with (004) preferred orientation. TiO₂ lattice parameters are retrieved from the 2θ peak positions and the Bragg formula. The computed values are a = b = 0.377 nm and c = 0.949 nm. They are slightly smaller than a₀, b₀ and c₀ from the JCPDS card, suggesting TiO₂ films under compressive stress.

Fig. 2 SEM images of differently thick TiO₂ overlayers deposited on quartz slides (see Fig. 1A) using reactive magnetron sputtering in the DC mode: (A) 20 nm TiO₂, (B) 50 nm TiO₂, (C) 100 nm TiO₂, (D) 200 nm TiO₂

Fig. 3 XRD patterns of as-deposited 200 nm-thick TiO₂ on quartz slide before (a) and after thermal annealing at 600 °C for 72 h (b).

To determine the optical properties of the TiO₂ material, optical transmission spectra of the different quartz/TiO₂ interfaces were recorded in air (Fig. 4A). TiO₂ films with visible range optical transmittance have shown higher photocatalytic effect than those with UV range optical transmittance.^37–41 The transmission measurements show almost zero transmission and thus high absorption for energies higher than about 4.3 eV for a 20 nm thick TiO₂ film and higher than 3.5 eV (355 nm) for a 200 nm thick TiO₂ overlayer. At shorter wavelengths, in region of strong absorption, the transmittance of the film T can be expressed in terms of the absorption coefficient α and the film thickness d, as follows:^39,42,43

ln(T) ≈ −α d (1)

Fig. 4 (A) Optical transmission spectra of TiO₂ thin films of different thicknesses deposited on quartz substrates (after correction of the substrate transmittance). (B) Optical absorption coefficient (αhν)^1/2vs. photon energy (hν) and the band gap widths of the TiO₂ films.

Taking into account the indirect-allowed transition in the TiO₂ band structure,⁴⁴ the absorption coefficient may be expressed by:

(αhν)^1/2 = A (hν − E_g) (2)

where hν is the photon energy, A is a constant independent of photon energy and E_g is the optical band gap of TiO₂. By extrapolating the linear part of (αhν)^1/2versus energy, the E_g value is determined for (αhν)^1/2 = 0 (Fig. 4B). As a result, the band gap widths equal to E_g = 3.30 ± 0.03 eV for 200 nm, 100 nm and 50 nm thick TiO₂ films, and E_g = 3.20 ± 0.02 eV for 20 nm thick TiO₂ films. The E_g values of the samples are close to the expected value of anatase phase (3.2 eV).⁴⁵ The exponential-type Urbach tails are observed on each plot of (αhν)^1/2 at lower energies, which arise from impurity perturbations near the conduction band (Fig. 4B).⁴⁴ In addition, optical interferences, most likely Fabry–Perot cavities, can be observed for TiO₂ overlayers with film thicknesses higher than 20 nm.

3.2 Characterization of quartz/Au NSs/TiO₂

Fig. 5A shows the morphology of the Au NSs formed by thermal deposition of a 4 nm thick gold film on quartz and post-annealing at 800 °C for 10 min. The dewetting of the gold film on the quartz substrate results in the formation of Au NSs with an average diameter d = 21 ± 9 nm, height h = 20 ± 6 nm and interparticle distance a = 9 ± 4 nm as described recently by ourselves.²⁷ TiO₂ films of 20–200 nm thickness were deposited onto the quartz/Au NSs using reactive magnetron sputtering in the DC mode and post-annealed at 600 °C for 72 h. The SEM image of a quartz/Au NSs coated with 50 nm thick TiO₂ is shown in Fig. 5B. The morphology and the size of the TiO₂ crystallites are comparable to those observed in Fig. 2 in the absence of the Au NSs.

Fig. 5 SEM image of Au NSs deposited on quartz by thermal evaporation of 4 nm thick gold films followed by post-annealing at 800 °C for 10 min before (A) and after coating with 100 nm thick TiO₂ film (B). (C) UV-vis transmission spectra of quartz/Au NSs/TiO₂ interfaces in water with different TiO₂ overlayer thicknesses: 0 (), 20 (), 50 (), 100 (), 200 nm ().

The presence of Au NSs beneath the TiO₂ overlayer results in different optical spectra compared to TiO₂ alone. Absorption UV-Vis spectra of quartz/Au NSs interfaces with differently thick TiO₂ overlayers are displayed in Fig. 5C. The optical signals obtained in the visible (λ = 420–800 nm) are due to the plasmonic response of the Au NSs. For a quartz/Au NSs interface (without TiO₂ coating), the plasmonic band is seen at λ_max = 520 nm. The presence of TiO₂ overlayers shifts λ_max to higher wavelengths in accordance with UV-vis studies on other lamellar LSPR interfaces (Table 1).^22,25–29 The plasmonic band oscillates between 520–685 nm depending on the thickness of the TiO₂ overlayer. Such an oscillation arises from the interaction of Fabry–Perot modes established inside the dielectric with the plasmon of the Au NSs.³⁴ At wavelengths below ≈400 nm, the sharp rise in the optical signal is due to the absorption of TiO₂.

Table 1 Determined LSPR λ_max in water for glass/Au NSs/TiO₂ interfaces as a function of TiO₂ overlayer thickness

TiO2 thickness (nm)	λ max (nm)
0	520
20	623
50	685
100	632
200	674

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3.3 Photodegradation of rhodamine B

In aqueous solutions, the photoassisted photocatalytic degradation of organic molecules is caused by the active species produced on the surface of the TiO₂ semiconductor. Under irradiation at a wavelength of appropriate energy, electron–hole pairs are formed with a lifetime that enables participation in chemical processes. The electrons in the conduction band of TiO₂ can reduce molecular oxygen to superoxide anions (O₂˙⁻), producing further hydroxyl and hydroxyl peroxide radicals (HO˙, HO₂˙),³ which oxidize the organic dye.⁴⁶ In parallel, the reaction of holes with water generates hydroxyl reactive species resulting in the direct oxidation of the dye at the valence band. To evaluate the photocatalytic activity of the different interfaces towards the oxidative degradation of organic molecules, photocatalysis experiments were carried out using rhodamine B (RhB) as a model molecule. An aqueous solution of RhB displays a strong and characteristic absorption band at 554 nm with a shoulder at 520 nm. The photocatalytic performance was monitored by the decay of the absorption of the dye as a function of irradiation time by UV-vis spectroscopy. Fig. 6 shows the change in the UV-vis spectrum of RhB (initial concentration of 5 × 10⁻⁶ M, 2 mL) aqueous solution in the absence (Fig. 6A) and in the presence of quartz/TiO₂ (100 nm thick) (Fig. 6B) photocatalyst under UV light irradiation. In all cases, the characteristic absorption band of tetraethylated RhB at 554 nm decreases during irradiation time. In the presence of quartz/TiO₂ (100 nm thick) photocatalyst, the oxidation is almost complete after 150 min irradiation (Fig. 6B). In a control experiment, direct irradiation of RhB (in the absence of photocatalyst) showed less than 20% degradation after 150 min under UV irradiation (Fig. 6A).

Fig. 6 UV/Vis spectra of an aqueous solution of RhB (5 μM) upon irradiation at λ = 365 nm, P = 0.5 W cm⁻² for 0 (black), 60 (blue) and 150 min (red) in the absence (A) and in the presence (B) of quartz/TiO₂ (100 nm thick TiO₂ film) photocatalyst.

The influence of the TiO₂ thickness on the photodegradation rate of RhB was estimated by following over time the change in the intensity of the absorption band of RhB at 554 nm under UV or visible light illumination in the presence of the different quartz/TiO₂ interfaces (Fig. 7). The photodegradation process can be fitted to a first order rate law by plotting ln(A₀/A) vs. time, where A₀ is the initial concentration and A the concentration upon irradiation at a given time. As seen in Fig. 7, direct photolysis of RhB under UV light irradiation is slow with an estimated k_UV = (0.3 ± 0.1) × 10⁻³ min⁻¹ (Table 2). The highest photodegradation rate is obtained for 100 nm thick TiO₂ films with k_UV = (7.3 ± 0.3) × 10⁻³ min⁻¹. As the optical band gaps of the different TiO₂ films are comparable (E_g ≈ 3.25 eV) the better photocatalytic behavior might be due to the presence of fewer recombination-induced defects in the material.

Fig. 7 Photocatalytic degradation of RhB (5 μM) upon UV (λ = 365 nm) (A) and visible (λ > 420 nm) light irradiation (B) in the presence of quartz/TiO₂ photocatalysts of different TiO₂ film thicknesses: 0 (), 20 (), 50 (), 100 (), 200 nm ().

Table 2 First order rate constants for the photodegradation of RhB using different photocatalysts

	k UV/min^−1 (×10^−3)	k Vis/min^−1 (×10^−3)
Without photocatalyst	0.3 ± 0.1	0.6 ± 0.1
Photocatalyst
quartz/20 nm TiO2	4.5 ± 0.3	2.0 ± 0.4
quartz/50 nm TiO2	5.3 ± 0.2	2.1 ± 0.4
quartz/100 nm TiO2	7.3 ± 0.3	3.5 ± 0.3
quartz/200 nm TiO2	2.0 ± 0.4	2.9 ± 0.2
quartz/Au NSs	0.6 ± 0.1	0.5 ± 0.1
quartz/Au NSs/20 nm TiO2	0.6 ± 0.1	0.7 ± 0.1
quartz/Au NSs/50 nm TiO2	7.3 ± 0.1	2.9 ± 0.3
quartz/Au NSs/100 nm TiO2	11.1 ± 0.1	3.3 ± 0.2
quartz/Au NSs/200 nm TiO2	0.4 ± 0.1	2.1 ± 0.3

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The photodegradation rate under visible light irradiation was additionally investigated. Direct visible light irradiation of RhB (in the absence of photocatalyst) resulted in less than 15% degradation after 150 min irradiation. This is slightly higher than the 10% obtained under UV irradiation under otherwise identical experimental conditions. The photodegradation rate under visible light irradiation in the presence of the quartz/TiO₂ photocatalyst was in most cases half of that under UV light. The band gaps of the different interfaces are located in the UV range (376–388 nm) which explains the rather low photodegradation rate under light irradiation with λ > 420 nm. The 100 nm thick TiO₂ films showed the best photocatalytic degradation rate in the visible region with k_Vis = (3.5 ± 0.3) × 10⁻³ min⁻¹. It is to be noted that the 200 nm thick TiO₂ film exhibited slightly improved photodegradation efficiency under visible light irradiation as compared to UV illumination.

In addition, the influence of plasmonic metallic nanostructures beneath the TiO₂ films on the photocatalytic performance was investigated. Indeed, RhB absorbs at a wavelength of λ ≈ 554 nm (Fig. 5C), which is located close to the plasmonic band of the quartz/Au NSs interface and at the peak tail of the quartz/Au NSs/TiO₂ interfaces (Table 1). Fig. 8 shows the change of the absorption intensity of RhB in the presence of quartz/Au NSs/TiO₂ interfaces under UV (Fig. 8A) and visible (Fig. 8B) light irradiation. No significant photocatalytic effect of the naked Au NSs on the photodegradation of RhB is seen (Table 2), even though the plasmonic band is located at λ_max = 520 nm, close to the absorption maximum of RhB. Table 2 summarizes the kinetic rate constants determined for the different systems. It becomes clear that the presence of Au NSs in most cases significantly diminished the photocatalytic efficiency of the oxidation of RhB under UV light irradiation. Only in the cases of 50 and 100 nm thick TiO₂ layers were significant enhancements of the photocatalytic activity recorded with k_UV = (7.3 ± 0.1) × 10⁻³ and (11.1 ± 0.3) × 10⁻³ min⁻¹, respectively. The better photocatalytic properties of the quartz/AuNSs/100 nm TiO₂ interface seem to be a result of the combined effect of the better photocatalytic properties of the TiO₂ film and the shape and position of the plasmonic band. Indeed, the close position of the plasmonic band, λ_max, of the photocatalytic interfaces in comparison to the absorption wavelength of RhB results in a resonance effect. It has already been shown that resonant conditions are favorable for enhancing surface fluorescence.²⁹ While quartz/Au NSs/20 nm TiO₂ fullfills these conditions better than the other interfaces, the UV-vis spectra of the 50 and 100 nm thick TiO₂ show increased absorption intensities at 554 nm, due to the broad plasmonic signal and seem to influence positively the photodegradation process of RhB.

Fig. 8 Photocatalytic degradation of RhB (5 μM) upon UV (λ = 365 nm) (A) and visible (λ > 420 nm) (B) irradiation in the presence of quartz/Au NSs LSPR interfaces coated with 0 nm (), 20 (), 50 (), 100 () and 200 nm () thick TiO₂ layers. For comparison, the results of direct irradiation of RhB in the absence of photocatalyst () are included.

The presence of the plasmonic structures did not improve the photocatalytic activity of the interface under visible light irradiation (Table 2). When compared to quartz/TiO₂ of comparable TiO₂ thickness, the presence of Au NSs even had a negative effect on the quartz/Au NSs/20 nm TiO₂ substrate. The interest of such hybrid interfaces is mostly for photocatalysis under UV light irradiation.

4 Conclusion

A novel strategy for the development of thin film TiO₂ photocatalytic interfaces was developed. It is based on the deposition of ultrathin TiO₂ films on quartz substrates using DC cathodic sputtering. A 100 nm film showed pronounced photocatalytic activity under UV illumination. A faster photodegradation of RhB (about 35% higher) was obtained in the presence of Au NSs beneath the 100 nm TiO₂ film under UV light illumination, taking advantage of the resonance effect between the absorption wavelength of RhB and the plasmon of the interface. These interfaces are very easy to prepare, highly stable to surface degradation and would be an alternative to other thin film-based photocatalysts. The results provided in this work hold promise in view of various photocatalytic applications.

Acknowledgements

The Centre National de la Recherche Scientifique (CNRS) and the Nord-Pas-de Calais region are gratefully acknowledged for financial support.

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